Colloidal dispersions of water-insoluble materials in aqueous-based carriers have found widespread use in a number of important applications including; paints, inks, agriculture, pharmaceutical, and construction chemicals, just to name a few. A colloidal dispersion is herein defined as a system in which finely divided particles, which are approximately 10 to 1,000 nanometers in size, are dispersed within a continuous liquid carrier in a manner that prevents them from being easily agglomerated or rapidly settled. In recent years, nanoparticle systems, generally defined as particles having diameters less than about 100 nanometers, have become increasingly important. In many of these systems the preferred carrier for the particles is water or a mixture of water and water-miscible co-solvents. An important performance feature of all of these colloidal systems is their inherent particle stability over the useful life of the system.
A great deal of exploration has been conducted to design aqueous-based colloidal dispersions such as pigment-based inks that are stable against agglomeration and settling for long periods of time. “Colloidally stable” as used herein means that the desired particle size or distribution of sizes of the colloid is relatively unchanged with time or storage conditions. Colloidal instability can occur by a number of phenomena such as: ripening, coalescence, agglomeration, or flocculation. One preferred approach to colloidal stability has been the use of small molecule ionic or nonionic surfactants. However, these systems have limitations based on the complexity of the aqueous carrier and associated additives contained therein.
A second approach has been the use of polymeric dispersants. In general, polymeric dispersants have been designed to have portions of the dispersant that are hydrophobic and portions of the dispersant that are hydrophilic. Polymeric dispersants are generally defined as random, block, or graft polymers and can have a wide variety of conformations and chemical compositions that are tailored to the specific surface of the water-insoluble colloid and the make-up of the aqueous-based carrier fluid.
One commercially important class of water-insoluble colloids is pigments. Pigments can generally be defined as inorganic or organic. Examples of inorganic pigments include metal oxides such as titanium dioxide, silicon dioxide, and numerous others. Examples of organic pigments are those commonly used as colorants in the art of ink technology, for example, quinacridones, phthalocyanines, azos, carbon blacks, and numerous others. Many polymeric dispersant systems have been proposed to disperse pigments in aqueous-based carriers. However, the surfaces of pigments vary substantially and will sometimes have dramatically different affinity for a given polymeric dispersant. The use of water-miscible organic co-solvents in the water phase of the colloidal dispersion may also have a profound impact on the affinity of a polymeric dispersant for a surface of a pigment.
Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught, and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment to a sufficiently small particle size, and to provide sufficient colloidal stability to the particles.
One approach to dispersing a pigment in an aqueous carrier involves mechanical milling of a pigment in the presence of a polymeric dispersant. Examples of polymer dispersants for milling a pigment are described in U.S. Pat. Nos. 6,245,832; 5,085,698; and 4,597,794; and US Publication Numbers 2006/0014855; 2007/0043144; and 2007/0043146. More recent copolymer dispersant designs disclose the use of a hydrophobic portion combined with multiple types of hydrophilic portions, as disclosed in U.S. Publication No. 2006/0084720 and EP 1666547.
Although these polymeric dispersants are known as colloidal stabilizers for pigments in water, they can be insufficient stabilizers of the pigment surface in a mixture of water and organic co-solvents. This is especially true under accelerated keeping conditions such as temperatures in excess of 40 degrees Celsius. The presence of such organic co-solvents, as is the case in most modern inkjet ink applications, can have deleterious consequences that result in an increase in the population of large particles or agglomerates of particles in the colloidal suspension or ink. Specific examples of the negative consequences of particle growth include: plugging of a small diameter inkjet printhead nozzle, loss of gloss in a printed image, and sedimentation of the colloid during storage in an inkjet ink tank or cartridge.
A second approach to dispersing a pigment with a polymeric dispersant involves covalently bonding the polymer to the pigment surface, or encapsulating the pigment surface with a polymer. Numerous approaches to this method of pigment stability have been proposed including, U.S. Pat. Nos. 5,990,202; 6,635,693; 6,972,303; and US Publication No. 2006/0155006. However, these approaches involve carefully controlled polymerizations in the presence of a pigment or approaches that involve volatile solvents that must be removed at some stage in the process. The complexity and added processing steps of such approaches add undesirable costs to the manufacturing process of a colloidal suspension or ink.
There remains the need for a simple means to produce a colloidal dispersion of water-insoluble pigment particles in an aqueous-based carrier that results in a distribution of particles that is stable to changes in particle size with time, especially under harsh conditions of high temperature and/or organic co-solvent formulations. Additionally, there remains the need for a polymeric dispersant approach that works well with many different types or classes of pigments.